gnition
consists in behaving in the same way when a stimulus is repeated as we
behaved on the first occasion when it occurred. This seems to be the
exact opposite of the truth. The essence of recognition is in the
DIFFERENCE between a repeated stimulus and a new one. On the first
occasion there is no recognition; on the second occasion there is. In
fact, recognition is another instance of the peculiarity of causal laws
in psychology, namely, that the causal unit is not a single event, but
two or more events Habit is the great instance of this, but recognition
is another. A stimulus occurring once has a certain effect; occurring
twice, it has the further effect of recognition. Thus the phenomenon
of recognition has as its cause the two occasions when the stimulus has
occurred; either alone is insufficient. This complexity of causes
in psychology might be connected with Bergson's arguments against
repetition in the mental world. It does not prove that there are no
causal laws in psychology, as Bergson suggests; but it does prove that
the causal laws of psychology are Prima facie very different from those
of physics. On the possibility of explaining away the difference as due
to the peculiarities of nervous tissue I have spoken before, but this
possibility must not be forgotten if we are tempted to draw unwarranted
metaphysical deductions.

True memory, which we must now endeavour to understand, consists of
knowledge of past events, but not of all such knowledge. Some knowledge
of past events, for example what we learn through reading history, is
on a par with the knowledge we can acquire concerning the future: it
is obtained by inference, not (so to speak) spontaneously. There is
a similar distinction in our knowledge of the present: some of it is
obtained through the senses, some in more indirect ways. I know that
there are at this moment a number of people in the streets of New York,
but I do not know this in the immediate way in which I know of the
people whom I see by looking out of my window. It is not easy to state
precisely wherein the difference between these two sorts of knowledge
consists, but it is easy to feel the difference. For the moment, I shall
not stop to analyse it, but shall content myself with saying that, in
this respect, memory resembles the knowledge derived from the senses.
It is immediate, not inferred, not abstract; it differs from perception
mainly by being referred to the past.

In regard to